1. Field of the Invention
This invention relates to an image reader such as an image scanner, a copying machine or the like that performs vertical scanning shading correction.
2. Description of the Related Art
Conventionally, an image reader has been used as an input device of an image scanner, a copying machine or the like, and recently a need for a high-quality image of the apparatus has been increased.
FIG. 13 is a sectional view of a conventional image reader, and FIG. 14 is a top view of the conventional one. As shown in FIGS. 13 and 14, the image reader comprises an image reader body 1, a document glass 2, a carriage 3, a supporting member 4, a shaft 5, a drive transmission member 6, a drive pulley 7, a follower pulley 8, a driving motor 9, a follower pulley supporting member 10, a spring member 11, a horizontal scanning white reference board 16, and a vertical scanning white reference board 17.
The document glass 2 is used to set a document manually. The carriage 3 is used to scan and read the document, and the supporting member 4, which contains bearings or the like, is fixed to the carriage 3. The shaft 5 supports the carriage 3 via the supporting member 4, and movement of the carriage 3 is restricted to a vertical scanning direction by the shaft 5. The drive transmission member 6, such as wire, a belt and the like, transmits a driving force to the carriage 3. The carriage 3 is connected to the drive transmission member 6, which is engaged with the carriage 3 via the drive pulley 7 and the follower pulley 8. The drive pulley 7 is connected to the driving motor 9 via a connecting shaft and a reduction mechanism (both not shown), and the carriage 3 is driven by rotating the driving motor 9. The follower pulley 8 is energized via the follower pulley supporting member 10 by the spring member 11, giving tension to the drive transmission member 6. The horizontal scanning white reference board 16 and the vertical scanning white reference board 17 shown in FIG. 14 are both fixed on the document glass 2 within a range readable by a line image sensor 14 described later.
FIG. 15 is a configuration diagram which shows an optical system of the conventional image reader. Since the document glass 2, the carriage 3, and the drive transmission member 6 in FIG. 15 are the same as those in FIG. 13, the same reference numerals are used for them and the description is omitted here. A light source 12 irradiates a document, and a reflecting mirror 13 reflects a reflected light from the document. The line image sensor 14 converts image information into electric signals, and an imaging lens 15 forms an image of a document on the line image sensor 14.
FIG. 16 is a block diagram showing an image reading circuit of a conventional image reader. In FIG. 16, a driver 18 controls the line image sensor 14, which outputs image signals R, G and B for every line each time a line synchronizing signal LS is inputted. Image signals R, G and B, corresponding to each pixel received by the line image sensor 14 are outputted in synchronization with a pixel clock CK. Amplifiers 19, 20 and 21 amplify levels of pixel signals of image signals, R, G and B outputted from the line image sensor 14, respectively, and output the amplified image signals R, G and B. A CPU 25 controls the driver 18 by using an image range signal a, and controls gains from the amplifiers 19, 20 and 21 via signal lines 22, 23 and 24 which transmit amplifier control signals r, g and b, respectively. The image range signal a is a signal indicating a period in which an image signal is actually outputted in synchronization with a line synchronizing signal LS. The CPU 25, which contains at least 3 or more channels of A-D converters, converts image signals R, G and B outputted by the amplifiers 19, 20 and 21, respectively, into digital signals. Also, the CPU 25, containing at least 3 or more channels of D-A converters, converts digital amplifier control signals into analog amplifier signals r, g and b, so as to control the amplifiers 19 to 21 as described above. A memory 26 is connected to the CPU 25. The CPU 25 controls the entire image reader facilities, and for example, controls the rotation of the driving motor 9 and turning-on of the light source 12.
The operation of the conventional image reader having the above configuration will be described below by using FIGS. 13 to 17. FIG. 17 is a positional relationship diagram showing the positional relationship between the vertical scanning white reference board 17 and the line image sensor 14. First, immediately after the image reader is turned on, or at an appropriate time when the image reader does not perform a reading operation, if necessary, the CPU 25 rotates the driving motor 9 so as to move the carriage 3 to a position in which the horizontal scanning white reference board 16 is readable and turns on the light source 12. When the light source 12 is turned on, a reflected light from the horizontal scanning white reference board 16 is reflected back by the reflecting mirror 13, and an image is formed on the line image sensor 14 by the imaging lens 15. The analog signals R, G and B obtained from the line image sensor 14 are converted into digital signals through the A-D converter in the CPU 25, and transferred to the memory 26. The CPu 25 reads the values transferred to the memory 26 and specifies values for amplifier control signals r, g and b so that the levels of the image signals R, G and B reach predetermined levels, respectively. The specified amplifier control signals r, g and b are converted from digital signals into analog signals through the D-A converter of the CPU 25, and transmitted to the amplifiers 19, 20 and 21, and the gains of the amplifiers 19, 20 and 21 are adjusted.
Next, after receiving an instruction to read a document sent from an external host computer (not shown), the CPU 25 drives the carriage 3 as well as turning on the light source 12. When the carriage 3 arrives at a start position for reading a document, a read operation is started. Then, the image signals R, G and B obtained from the line image sensor 14 are converted to digital signals through the A-D converter in the CPU 25, and after being subjected to image processing, if necessary, the signals are sequentially transferred, directly or via the memory 26, into the external host computer.
Next, vertical scanning shading correction is described below. In general, an LED, a fluorescent or a halogen lamp is used as a light source for an image reader. If fluctuation of the amount of light or the spectrum of the light source 12 during a document read operation is smaller than a quantization step of the A-D converter, signal levels after A-D conversion obtained from the vertical scanning white reference board 17 are constant, and therefore the tone effects of image data obtained from the document are kept constant. If the tone effects of the colors R, G and B are multi-tone such as 256 tones, however, fluctuation of the amount of light and the spectrum of the typical light source described above may be greater than the quantization step of the A-D converter, and therefore the signal level during a document read operation varies according to the fluctuation of the amount of light and the spectrum of the light source.
In the vertical scanning shading correction, fluctuation of the signal level caused by fluctuation of the amount of light and the spectrum of the light source is corrected so as to secure the tone effects of the image data. In general, the number of pixels of the line image sensor 14 is greater than that of a readable document size. By using the extra pixels, data on the vertical scanning white reference board 17 are read to obtain image signals to be used in the vertical scanning shading correction. At this time, the positions of pixels on the line image sensor 14 to be used in the vertical scanning shading correction are determined by a layout such as a document reading range. First, the CPU 25 stores, in the memory 26, signal levels Ar, Ag and Ab of the colors R, G and B obtained from the vertical scanning white reference board 17 before reading a document. The signal levels Ar, Ag and Ab become reference signal levels of the vertical scanning shading correction. Next, during image read operation, the CPU 25 obtains signal levels Br, Bg and Bb from the vertical scanning white reference board 17 at constant intervals of time or distance, and specifies the values of the amplifier control signals r, g and b, respectively, so that the signal levels Br, Bg and Bb are the same as the corresponding signal levels Ar, Ag and Ab previously obtained. As described above, the specified amplifier control signals r, g and b are converted from digital signals to analog signals through the D-A converter of the CPU 25, and transmitted to the amplifiers 19 to 20, adjusting the gains of the amplifiers 19 to 21.
As described above, in the vertical scanning shading correction, feedback control is performed for adjusting the gains of the amplifiers 19 to 21, based on the signal levels obtained from the vertical scanning white reference board 17, by means of the line image sensor 14. Thus, the tone effects of image data obtained from a document can be secured.
The conventional image reader described above, however, has the following two problems in the vertical scanning shading correction.
A first problem is that a greater width in the horizontal scanning direction is required for the vertical scanning white reference board 17, whereby a greater size of an image reader is required. The width in the horizontal scanning direction required for the vertical scanning white reference board 17 will be described below.
As mentioned above, the number of pixels of the line image sensor 14 includes more pixels than the number of pixels corresponding to a readable document size though the number of excessive pixels is not so great. For example, the number of pixels of a line image sensor made by Toshiba, TCD2551D, is 5,340. Under this condition, assuming that the optical resolution of the image reader is 600 dpi and the readable document size is 216 mm (letter size), the required number of pixels is equal to 216.div.(25.4.div.600), that is, 5,102, whereby the number of excessive pixels equals 238 (10.08 mm) calculated by 5,340-5,102. However, all of 10.08 mm is not available to the vertical scanning shading correction. First, the read area of the document and that of the vertical scanning white reference board 17 cannot be positioned contiguously on the line image sensor 14, and an interval of about 4 mm is required between them. The interval is needed because, during scanning in the vertical scanning direction, the carriage 3 is not necessarily operated to perform scanning in parallel with the vertical scanning white reference board 17, but the scanning may be performed obliquely by about .+-.2 mm according to assembly precision, and because another 2 mm or so is required for mounting precision of the vertical scanning white reference board 17 and the document glass or for a margin for mounting with adhesive or the like. In addition, even if the precision of the mounted position of the line image sensor 14 is optically adjusted, about .+-.1 mm is required, and another .+-.1 mm or so is required for the precision of the mounted position of the vertical scanning white reference board 17. Furthermore, since the document glass 2, the light source 12, and the components arranged around the light source 12 have a certain degree of reflectance, a part of the light reflected by the light source 12 and then reflected by the vertical scanning white reference board 17 is reflected by the document glass 2, the light source 12, and the components around the light source 12, thus returning again to the vertical scanning white reference board 17, to intensify the apparent brightness of the vertical scanning white reference board 17. The apparent brightness of the vertical scanning white reference board 17, due to the influence caused by scanning obliquely by .+-.2 mm or so according to the assembly precision when the carriage 3 is operated to perform scanning in the vertical scanning direction as described above, fluctuates unless there is a certain width in the horizontal scanning direction of the vertical scanning white reference board 17. Therefore, in order to remove the fluctuation in the brightness of the vertical scanning white reference board 17, it is required to keep constant the apparent brightness, and to provide the vertical scanning white reference board 17 with an approximately 3 mm wider width in the horizontal scanning direction for an area, on which the vertical scanning shading correction is performed.
Thus, the area actually available to the line image sensor for the vertical scanning shading correction equals 10.08 mm-(4+1+1+3) mm, that is, 1.08 mm at an end. Therefore, the width required of the vertical scanning white reference board 17 in the horizontal scanning direction equals, as shown in FIG. 17, the sum of a width of an area A available to the line image sensor 14 for the vertical scanning shading correction, a width of an area B obliquely scanned, an allowance C for the precision of the mounted position of the line image sensor 14 or the vertical scanning white reference board 17, and a width D required to keep the apparent brightness constant. In the example above, the result of the formula above is 1.08 mm+2.times.(2+1+1+3) mm=15.08 mm, and therefore the size in the horizontal scanning direction of the document glass 2 and the length of the light source 12 become greater, thus leading to a larger size of the image reader. In FIG. 17, to simplify the explanation, the line image sensor 14 is directly associated with the vertical scanning white reference board 17. It, however, applies to an optical system which is an equal-magnification optical system, and therefore for a reduction optical system, it is assumed that data are converted to data based on either of scales of the document or of the image (the line image sensor 14). Also, in order to perform the vertical scanning shading correction, it is necessary for the line image sensor 14 to read not only the document, but also the vertical scanning white reference board 17 by forming an image on a position different from that of the document. However, in a typical line image sensor, the positional precision of read pixels for a foot of a lead frame, which is a reference for mounting, is .+-.0.8 mm or so, which is not so high. The value of 0.8 mm is a size in the line image sensor 14. Assuming that the magnification of the optical system is 0.2, it amounts to .+-.4 mm (.+-.0.8.div.0.2=.+-.4 mm) on a document.
Next, a second problem will be described. In the vertical scanning shading correction, the carriage 3 is operated to perform scanning so that the line image sensor 14 reads the vertical scanning white reference board 17. If there is dirt or the like on the vertical scanning white reference board 17 or a portion of the document glass 2 near the vertical scanning white reference board 17, the signal levels Br, Bg and Bb, which are referred to during the vertical scanning shading correction, fluctuate due to the dirt. Based on the signal levels Br, Bg and Bb, the CPU 25 specifies the values of amplifier control signals r, g and b so as to adjust the gains of the amplifiers 19, 20 and 21. Therefore, the CPU 25 has a problem of losing tone effects of image data obtained from the document due to the vertical scanning shading correction.
An image reader which can precisely read the tone effects of a document and which can be downsized would be desirable.